Methods of fabricating integrated transducer assemblies

ABSTRACT

A transducer assembly comprises a thin diaphragm of silicon having at least one piezoresistive element located in a central active area of the diaphragm. The diaphragm has a metal ring containing terminal accommodating apertures about the periphery thereof. The metal ring bounds the outermmost portion of the diaphragm designated as a non-active area. An annular housing fabricated from a high dielectric material has a series of wire accommodating grooves in a side wall thereof each one aligned with a predetermined one of said terminal accommodating apertures. The metal ring is secured to the annular housing with the active area of the diaphragm surrounded by a central aperture of the annular housing. The metal ring is secured to the housing by an electrostatic glass bond.

This is a divisional of application Ser. No. 393,804, filed on Sept. 4,1973, now U.S. Pat. No. 3,873,956.

BACKGROUND OF INVENTION

This invention relates to electromechanical transducers and moreparticularly to such transducer assemblies employing piezoresistivesemiconductor strain gages on thin diaphragms which are secured to ahousing by electrostatic glass bonds.

The invention further relates to techniques for fabricating an improvedtransducer assembly employing glass bonds.

There is a class of pressure transducers which utilize thepiezoresistive effect to enable high outputs. These units aresemiconductors and have resulted in the construction ofelectromechanical force transducers with superior output characteristicsand operating frequencies as compared to those of the prior art.

The piezoresistive transducer technology is compatible in many respectswith integrated circuit techniques.

Generally, a transducer comprises a relatively thin diaphragm which maybe constructed of silicon. A strain gage bridge assembly is thendiffused or grown onto the silicon diaphragm as are suitable contactarrangements. The diaphragm plus the gages and contacts are then mountedto a suitable housing in order to properly protect and utilize thetransducer assembly.

It is desirable to secure the diaphragm to the housing so that ahermetic seal is obtained. This assures protection of the gages andenables accurate and reliable measurements while providing better hightemperature operation.

The prior art shows various schemes for formulating a bond between thediaphragm and the housing. Thus, epoxy bonds are used. These bondssuffered as the epoxy exhibited plastic behavior at raised temperaturescausing spurious forces to be applied to the gage via the diaphragm.

Glass bonds were also used and are known. One could secure the diaphragmto a housing be means of solder glass. These glasses devitrify at atemperature compatible with methods of lead attachment to silicon. Upondevitrification of these glasses, a partially crystalline structuredevelops which results in a seal much stronger and harder than availablewith a vitreous glass. The difficulty with these techniques is that thebond was not uniform and the sealing process is not compatible withcertain fabrication processes employed in the fabrication ofpiezoresistive sensors. Another problem was that the gages weresubjected to a compressive force during the cooling process.

The prior art recognized that strong hermetic seals could be acheivedbetween glass and various metals at relatively low temperatures byapplying an electrostatic bias across the glass to the metal interfaceduring the seal process. Such seals were achieved at temperaturesbetween 200°F to 400°F below the melting point of the glass. In anyevent, this technique still presented problems when attempting tofabricate high quality transducers.

For example, U.S. Pat. No. 3,654,579 entitled "ELECTROMECHANICALTRANSDUCERS AND HOUSING", issued on Apr. 4, 1972 to A. D. Kurtz, et.al., and assigned to the same assignee herein, shows an improvedtransducer. The transducer employs a semiconductor wafer secured to ahousing by means of a bond. In any event, the housing has wireaccommodating apertures or slots. These slots enable one to direct wiresor leads therethrough, which leads interface with the contacts orterminals areas on the silicon diaphragm. As is explained in the patent,by routing the leads through the apertures, one obtains improvedoperation and better characteristics in regard to the transducingassembly. Hence, it is extremely desirable to maintain a "slotted"housing in the production of a high performance transducer.

It is also important that the edges of the diaphragm be well bonded orsealed to the housing since a simply supported diaphragm has asignificantly lower sensitivity than a clamped diaphragm. In addition,as above indicated, the seal or bond must exhibit good mechanicalproperties as well as being leak-free over the full temperature range ofthe transducer.

Due to the fabrication process, a "step" is formed in the clampingregion of the diaphragm which would otherwise prevent the formation of aproper seal. Therefore, due to these considerations as well as terminalplacement on the diaphragm, one experiences difficulties with theelectrostatic bonding technique, even though the technique can afford agood seal.

It is therefore an object of the present invention to provide animproved transducer assembly employing a slotted housing with a silicontransducer secured to the housing by means of a glass bond formed by anelectrostatic process.

DESCRIPTION OF PREFERRED EMBODIMENT

A tranducer assembly comprises a thin diaphragm fabricated from asemiconductor material and having located on a surface thereof at leastone force responsive element, said element positioned within a centralportion of said diaphragm, a metal ring having a central aperturedefining said active area and a plurality of terminal accommodatingapertures about the periphery of said ring, means securing said ring tosaid diaphragm with the active area within said central aperture of saidring and an high dielectric annular housing having a series of wireaccommodating grooves in a sidewall thereof secured to said ring withthe apertures in said ring aligned with the grooves in said housing.

Techniques for bonding the ring to the housing to provide a hermeticseal are also described.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a view in perspective illustrating a typical transducerassembly employing a relatively thin diapharagm, a bridge assembly andcontacts.

FIGS. 2A to 2H are a series of views showing the structures obtained bya series of steps of a process which is used to fabricate a transducerassembly according to this invention.

FIGS. 3A and 3B are respectively a top and a front view of alongitudinal tubular housing or an annular housing assembly according tothis invention.

FIG. 4 is a side view of a transducer assembly depicting a transducerbonded to a housing by means of a glass electrostatic bond.

DETAILED DESCRIPTION OF FIGURES

Referring to FIG. 1 there is shown a silicon diaphragm 20 containing afour-arm wheatstone bridge assembly. The diaphragm 20 comprises a thindisk of silicon onto which piezoresistive bridge elements 10, 11, 12 and13 have been atomically bonded or otherwise deposited using conventionalsemiconductor techniques as solid state diffusion or epitaxial growthtechniques.

The strain gage configuration is determined by an oxide masking orphoto-lithographic technique. Each of the stress sensors or gages 10,11, 12 and 13 may be isolated from the silicon diaphragm 20 by theformation of a P-N junction and are arranged on the surface so that twoof the elements are in tension and two are in compression for theapplication of a force.

Shown within a dashed line enclosing the piezoresistive elements 10through 13 is an area 14. This area is designated as the active area andis that area which is primarily effected by the application of a forceto the diaphragm 20.

The piezoresistive elements 10 through 13 are arranged on the surface ofthe silicon diaphragm and within the active area 14 to take primaryadvantage of the forces applied thereto in relation to the semiconductorcrystallographic axis. Briefly, it is known that in the design of suchtransducers, cognizance must be taken of both the longitudinal andtransverse piezoresistive coefficeients if optimum characteristics areto result.

The peripheral area shown on the diaphragm is that area which is definedas the non-active area and is that area used to bond the diaphragm 20 toa housing.

In this manner, the terminal contacts 17, 18, 19, 22 and 23, which aremetal, are directed and located on the non-active area. It is to thesecontacts that leads will be soldered or attached to and directed to asuitable terminal assembly or connector associated with a housing. Ifreference is made to the above noted patent, examples of such housingswith wire or conductor accommodating channels will be shown.

Referring to FIG. 2A, there is shown a silicon wafer 30 fabricated froma monocrystalline N-type silicon. A layer of silicon dioxide 32 is grownon a surface of the silicon wafer 30 and to a depth of about 2,000Angstroms. The layer of silicon dioxide 32 can be grown under elevatedtemperature in the presence of water vapor by a thermal oxidationprocess.

The contact pattern and runners are then impressed on the waferutilizing silicon dioxide masking and photolithographic techniques anddepositing impurities to form ohmic contacts.

The contact areas as 17 and 19 and runners are formed (FIG. 2B) by themasking process and diffused into the etched-out area. FIG. 2B shows across section through a contact terminal. As is seen, there is a step 33provided which is equal to the height of the original oxide layer 32 orabout 2,000 Angstrom units. FIG. 2C shows a top view of the diaphragmwith an etched-out and diffused contact and runner area 34.

As indicated, the terminal area is located in the nonactive area of thewafer which is the area to which the diapharagm 30 is to be secured. Thestep 33 will prevent a good hermetic seal from ocurring because of thediscontinuity. The openings as 34, 35, 36 and so on correspond to eachterminal and runner or each contact as 17, 18, 19, 22 and 23 on thewafer shown in FIG. 1.

Referring to FIG. 2D, a thick layer of silicon dioxide 37 is grown ontop of the step and the layer 32. The time dependence of the growth ofthe layer of silicon dioxide is typified by a parabolic rate. Thisparabolic rate is typical of the reaction which is limited by theavilability of "molecular water" at the interface of the silicon. In anyevent, the silicon dioxide grows "faster" over the diffused contactareas than over the silicon dioxide layer 32. Therefore, a layer ofapproximately 16,000 Angstroms will almost completely eliminate the stepand only a slight depression 38 remains. This depression does notprevent the formation of a good hermetric seal.

After the thick oxide layer 37 is grown (FIG. 2D), the silicon dioxidelayer 37 is masked and etched and the piezoresistive bridge elements arediffused at the center of the diaphragm within the active area 14. Thisdiffusion is shown in cross section view in FIG. 2E and a top view inFIG. 2F. It is generally advantageous to perform the diffusion at thisstage after the growth of the thick step-leveling and insulating oxideover the contact runners. This is so since oxidation subsequent to theresistor diffusion results in a loss of control and the reduction of thenet average impurity concentration in the piezoresistive elements. It isoften desirable to have maximum impurity concentration and diffusion andperforming the sequence of operations in the sequence or steps indicatedallows this. The resistor pattern is caused to overlay the contactpattern to assume the structure of FIG. 1.

A piezoresistor 39 is shown in cross section in FIG. 2E as an exampleonly. After the resistors have been diffused into the diaphragm 30, theterminals are formed by a metalization process. The formation of metalterminals is known. Therefore metal is deposited on the contact areas asdesired.

As indicated previously, the runners are diffused and form ohmiccontact, the terminal areas are metalized so that a solder connectioncan be made to route leads and so on to a final connector assembly.

Referring to FIG. 2G, there is shown a top view of the metalizedstructure. The metal ring 40 is deposited aluminum and has a series ofU-shaped apertures as 41 which surround or enclose a terminal area onthe wafer. Each contact area is also metalized at the terminal portionof the contact near the periphery and within the non-active area. Thering 40 as shown as well as the metalized terminal areas as 47, 48 and49 correspond with 17, 18 and 19 of FIG. 1. The metal may be aluminumand because of the metalization techniques, the areas formed arerelatively flat and smooth.

Referring to FIG. 2G, the structure depicted is as follows. Thediaphragm 50 has diffused therein a bridge arrangement 51. The resistorsof the bridge are accessed by diffused runners (dashed) as 52. Therunners are directed to the periphery where they are coupled to a metalrod 49 which appears as a "dot." This is a terminal area 49 enabling oneto solder and so on. A metal ring 40 is also deposited and is insulatedby the thick layer of silicon dioxide which is on the surface 50. Themetal ring 40 as formed is smooth and flat and is used to mount thetransducer to a housing as will be explained in conjunction with FIGS. 3and 4.

Referring to FIG. 2H, there is shown an enlarged view of the ringaperture 41 of FIG. 2G. As is seen, the metal ring 40 has the aperture41. The metal ring 40 is, of course, deposited on the thick layer ofsilicon dioxide 37 and hence it is insulated from both the diaphragm 30and the ohmic runner or contact 50. The terminal area 48 is metalizedduring the same process as employed in formation of the metal ring 40and appears as a dot or circle of metal 48. This enables one to solderor bond a wire or conductor to the terminal area 48 and hence gainaccess to all the important nodes of the bridge assembly as shown inFIG. 1.

Referring to FIG. 3A there is shown a top view of glass annular housing60. The housing 60 may be fabricated from glass, quartz or other highdielectric material suitable for electrostatic bonding. An ideal glassfor this purpose is Corning Glass No. 7740. The housing may have acentral aperture 61 which is approximately congruent with the activearea of the wafer. This is, of course, necessary to permit the wafer toreadily deflect when secured to the housing. The housing also contains aseries of wire accommodating grooves or apertures 62. The apertures arealigned to coact with the apertures in the metal ring 40 of FIG. 2G.Thus, when the ring 40 is placed on the top surface of the housing 60and correctly oriented, each terminal as 47, 48 and 49 coacts with agroove 62.

Hence, a wire can be secured to the terminal and directed within thegroove. FIG. 3B shows a side view of the housing and clearly depicts thegrooves or slots 62.

Referring to FIG. 4 a processed transducer 70 (FIG. 2G) is placed on thetop surface of a housing 60. The active area of the diaphragm isenclosed within the opening of the housing 60 to permit easy deflectionof the diaphragm upon application of a force to the diaphragm. The waferis aligned with the housing such that the terminal areas 72, 73, and 74are located above the appropriate slots 75, 76, and 77 in the housing.The transducer 70 is placed with its metal ring 78 in contact with aglass housing 60. The non-active area as defined by the ring is to bebonded to the top of the housing 60 by using electrostatic bondingtechniques.

In any event, the electrostatic bonding techniques can provide a sealbetween the metal ring 70 and the glass housing 60. An electrostaticbias is applied between the wafer 70 and the housing 60. The bias isobtained from a high voltage source 80 capable of providing a voltage inexcess of 200 or more volts. The units brought in contact under atemperature of between 200°F to 400°F below the melting point of glassand the bias is provided. A strong mechanical bond is provided betweenthe metal ring 78 and the housing 60. The electrostatic bond 79 isextremely thin and the bond formed is extremely strong.

The bond can be conveniently employed because one is bonding a metalring 78 to the glass housing 60. The ring is smooth and flat and has no"step" transition, plus the conductivity is much lower than SiO₂ andhence a better bond is more easily afforded. The bond is referred to inthe prior art as an electrostatic bond or an anodic bond. Measurementsshow that such bonds may be on the order of 20 to 200 Angstroms thick.The heat is used to decrease the conductivity of the wafer and silicondioxide layers to cause current to flow through the same and the glassthus creating the bond.

This composite of the silicon disc with integral piezoresistive elementsand the glass housing comprise essentially a complete transducerstructure. In some cases, however, it may be advantageous to affix thisstructure to a metal housing. Such a metal housing may or may not havelead wire accommodating apertures, and the bond between the structuredescribed and this housing may be an electrostatic bond, solder glassbond, epoxy bond, etc., depending on the nature of the designrequirements. It is, of course, not necessary that a glass housing 60 befabricated as an individual piece. It may, in fact, be a layer formedupon a metal housing be conventional techniques. The important point isthat a desirable bond is formed between silicon and glass members viaelectrostatic bond. This technique affords improvement in performance aspreviously noted.

The use of a glass housing and metal ring to facilitate the bond is thepreferred embodiment of the invention. However, the technique oftwo-step oxidation can be employed to bond a silicon diaphragm directlyto a metal housing via an intermediate SiO₂ layer while eliminating theundesirable step typically formed with one-step oxidation.

While the foregoing description and specification sets forth theprinciples of the invention in connection with specific apparatus, it isto be understood that the description is made only by way of example andnot as a limitation of the scope of the invention as set forth in theaccompanying claims.

We claim:
 1. A method of fabricating a transducer comprising the stepsof:a. growing a relatively thin layer of silicon dioxide on a wafer ofsilicon, b. masking said silicon dioxide to provide a contact pattern,c. depositing impurities according to said pattern to form ohmiccontacts. d. growing a thick layer of silicon dioxide over said pattern,e. forming at least one piezoresistor on said wafer to cooperate withsaid contact pattern, f. forming metal terminals coupled to said contactpattern, g. metalizing a peripheral portion of said thick layer ofsilicon dioxide to form a surface to which any electrostatic glass bondcan be made.
 2. The method according to claim 1 wherein said relativelythin layer of silicon dioxide is about 2,000 Angstroms and said thicklayer is at least five times greater than said thin layer.
 3. The methodof fabricating a transducer comprising the steps of:a. growing a thinlayer of silicon dioxide on a diaphragm of silicon, b. etching saidlayer according to a contact pattern, c. diffusing a contact pattern onsaid diaphragm, said process causing an undesired transition to appearat said etched portion, d. growing a thick layer of silicon dioxide onsaid pattern to substantially eliminate said transition, e. etching anaperture in said thick layer of silicon dioxide to communicate with saiddiaphragm, f. forming a piezoresistor element in said wafer within saidaperture in cooperation with said contact pattern, and g. metalizing aperipheral area of said thick layer to provide a relatively smoothbonding surface for said diaphragm.
 4. A method of fabricating atransducer, comprising the steps of:a. growing a first layer of silicondioxide on a first surface of a silicon wafer, b. forming a contactpattern on said surface using a silicon dioxide masking technique, c.depositing impurities within said contact pattern to form ohmiccontacts, d. growing a thicker layer of silicon dioxide over said firstlayer and said contact pattern, e. etching said first and thick layer ofsilicon dioxide to form an opening therein, f. depositing at least onepiezoresistor element within said opening on said silicon wafer, g.forming terminals about said contact pattern by a metalizationtechnique, said terminals coupled to said element, h. depositing a metalring about the periphery of said thick layer of silicon dioxide withsaid terminals encircled by U-shaped apertures in said ring, i. glassbonding said diaphragm to an annular housing with said bond formedbetween said metal ring a surface of said housing.
 5. The methodaccording to claim 4 wherein:a. said first layer of silicon dioxide isabout 2,000 Angstroms thick.
 6. The method according to claim 5wherein:a. said second layer of silicon dioxide is about 16,000Angstroms thick.
 7. The method according to claim 4 wherein:a. saiddeposited metal ring is aluminum.
 8. The method according to claim 4wherein:a. said bond between said metal ring and said annular housing isa thin glass bond.
 9. The method according to claim 4 wherein saidannular housing has a series of grooves in said sidewall which arealigned with said U-shaped apertures in said ring, anda. directing leadsfrom said terminals encircled by said U-shaped apertures and within saidgrooves in said sidewall of said housing.